1. Field of the Art
The present invention relates in general to a method and an apparatus for discriminating a machining (cutting or grinding) state from a non-machining state in a machine tool, and more particularly to such method and apparatus for detecting machining actions of a cutting tool or grinder, based on vibrations produced while the workpiece is cut or ground by the tool.
2. Related Art Statement
In machine tools, particularly numerically controlled machine tools controlled by a numerical control system, actual machining actions of a cutting or grinding to machine a workpiece are detected in distinction from non-machining actions of the tool, so that the workpiece is machined based on the obtained information as to whether the tool is currently machining the workpiece or not. In this manner, a machining operation is effected automatically or unattendedly, for maximum machining efficiency.
For detecting actual machining or non-machining of a workpiece by a tool, there have been proposed several methods including that which uses a torque detector incorporated in a spindle drive system to sense a current dynamic torque in the drive system. In this method, a judgement is made as to whether the workpiece is actually cut or not, based on variations in the detected dynamic torque values. In another proposed method, a load current of a drive motor is measured and the machining on the workpiece are sensed according to variations in the measured load current. However, these methods for discrimination between the machining and non-machining actions or states in dependence on the detected torque or load current are not satisfactory in discrimination accuracy and response, and are not considered sufficiently capable of meeting practical requirements.
In the light of the above-indicated inconveniences, an alternative method has been proposed, as disclosed in Japanese Patent Applications which were laid open under Publication Nos. 57-173462 and 57-89751, wherein vibrations that are produced during a cutting operation on a workpiece are detected by a vibration sensor, so that the machining action is sensed based on the presence or absence of a signal from the vibration sensor. This method provides considerable improvements in accuracy and response in discrimination between the machining and non-machining actions.
While the above alternative method is practically satisfactory in terms of discrimination accuracy and response, it still suffers a problem that the vibration sensor may pick up external noises, which may cause erroneous control of the machine tool, because there are no provisions for distinguishing the picked-up external noises from vibrations that are generated due to machining actions of the tool on the workpiece.